In the compressor portion of an aircraft gas turbine engine, atmospheric air is compressed to 10-25 times atmospheric pressure, and adiabatically heated to 800°-1250° F. in the process. This heated and compressed air is directed into a combustor, where it is mixed with fuel. The fuel is ignited, and the combustion process heats the gases to very high temperatures, in excess of 3000° F. These hot gases pass through the turbine, where rotating turbine wheels extract energy to drive the fan and compressor of the engine, and the exhaust system, where the gases supply thrust to propel the aircraft. To improve the efficiency of operation of the aircraft engine, combustion temperatures have been raised. Of course, as the combustion temperature is raised, steps must be taken to prevent thermal degradation of the materials forming the flow path for these hot gases of combustion.
One well-known solution that has been undertaken to protect the metals that form the flow path for the hot gases of combustion have included application of protective layers having low thermal conductivity. These materials are applied as thermal barrier coating systems (TBCs), typically comprising a bond coat that improves adhesion of an overlying ceramic top coat, typically a stabilized zirconia. These systems are known to improve the thermal performance of the underlying metals that form the flow path in the hot section of the engine. However, as temperatures of combustion have increased, even these TBCs have been found to be insufficient.
Another solution that has been used in conjunction with TBCs is air cooling metal parts. Initially, impingement cooling provided a flow of air from the compressor to the back side of the metal parts comprising the gas flow path. As temperatures increased even further, serpentine passageways were formed in the metallic components and cooling air was circulated through the parts to provide additional cooling capability, the cooling air exiting through apertures positioned in the gas flow side of the component, providing an additional film layer along the gas flow path. Even though the air from the compressor is adiabatically heated to perhaps as high as 1250° F., the compressor air is still significantly cooler than the combustion gases moving along the gas flow path of the engine, and this air forms a barrier to protect the metal components from the hot combustion gases. However, as the temperatures of the combustion process have continued to increase, even these tried and true methods are reaching their limitations. The combustion temperatures are now sufficiently high that even the best superalloys coated in accordance with the prior art and outfitted with the well-known and elaborate cooling mechanisms exhibit shortened lives as a result of thermal degradation. In particular, the combustor liners of high efficiency, advanced cycle turbine engines are prone to failure as a result of thermal degradation.
While some modifications of the traditional flow path surfaces have been applied in the past, such as the application of materials over the TBC, these modifications have been directed to reducing the emissions of pollutants such as unburned hydrocarbons (UHC) and carbon monoxide (CO). One such modification is set forth in U.S. Pat. No. 5,355,668 to Weil et al., assigned to the assignee of the present invention, which teaches the application of a catalyst such as platinum, nickel oxide, chromium oxide or cobalt oxide directly over the flow path surface of the thermal barrier coating of a component such as a combustion liner. The catalyst layer, is applied to selected portions of flow path surfaces to catalyze combustion of fuel. The catalytic material is chosen to reduce air pollutants such as unburned hydrocarbons (UHC) and carbon monoxide (CO) resulting from the combustion process. The catalytic layer is applied to a thickness of 0.001 to 0.010 inches and is somewhat rough and porous, having a surface roughness of about 100-250 micro inches, in order to enhance the surface area available to maximize contact with the hot gases in order to promote the catalytic reaction. The rough surface assists in creating some turbulence that promotes contact the catalytic surface.
These prior art solutions are either directed to problems that are unrelated to the problem of high temperatures experienced by combustor walls, such as the Weil patent, or provide different solutions to the problem of high temperatures resulting from the combustion process. The present invention provides a different approach to the problem of high temperatures experienced by combustor walls.